Low-clearance antenna



ZJMLW Feb. 13, 1951 P. N. sELGxN Low-CLEARANCE ANTENNA Filed April l2,1947 INVENTOR PAW. J. SELGIN ATTORNEY llnwl Patented Feb. 13, 1951UNITED STATES PATENT OFFICE LOW-CLEARANCE ANTENNA n Paul J. Selgin, FortWayne, Ind., assignor, by mesne assignments, to Farnsworth ResearchCorporation, a corporation of Indiana Application April 12, 1947, SerialNo. 741,109

claims. (c1. 25o- 33) This invention relates to high frequency antennaeand particularly to an antenna for radiating a vertically polarized wavesubstantially uniformly in all horizontal directions.

It is conventional practice to establish radio communications between amoving vehicle such as a train and another train or a xed station. For aradio communications system of this type an antenna is required which isusually mounted on the roof of a vehicle such as a railroad car. Sincethe train must be able to pass under bridges or through tunnels of xedclearance, it is desirable to provide an antenna which has low overallvertical dimensions and a high efficiency comparable to that of aconventional half-wave dipole antenna. The horizontal radiation patterndeveloped by the antenna should preferably be substantially circular.

It has been suggested to provide an antenna comprising a lower cone andan upper disc. This so-called discone antenna has a very wide frequencyrange. However, the height of the conventional` discone antenna isapproximately a quarter wave length at the cuto" frequency which is toohigh for a low clearance antenna for vehiL cles. It has also beenproposed to provide a horn or radiating wave guide consisting of twoinverted `cones having their apices adjacent to each other. Anotherconventional antenna of this general type' has two radiating conescoupled to the two conductors of a coaxial transmission line. The 'twocones of the biconical antenna are of equal diameter, and the overallvertical height of the antenna is not low enough for the purposesreferred to. Furthermore, certain diiculties have been experienced witha biconical antenna.

should be adjustable While the antenna is in D operation so that wavesof different frequencies ,may be either radiated from or received by theantenna.

It is an object of the present invention, theretore. t0 provide ahishfrequency antenna which combines low vertical overall dimensions with anefficiency comparable to that of a conventional half-wave dipole. i

Another object of the invention is to provide a novel high frequencyantenna which will radiate a vertically polarized wave having asubstantially circular radiation pattern ina horizontal plane. n

A further object of the invention is to provide an antenna suitable forintercepting and radiating high frequency energy and having a low re-Aactive impedance with an impedance matching network which may beadjusted during operation of the antenna for properly matching theimpedance of the transmission line feeding the antenna to that of theantenna at a desired frequency.

In accordance with the present invention there is provided an antennaccmprising a first an.. nular antenna element and a second annular an-utenna element. There are further provided la transmission line andimpedance means for coupling the transmission line to the two antennaelements. The rst antenna element has a diameter which is smaller thanthat of the second an'- tenna element. Preferably the second or lowerantenna element has a diameter of the order of one half the operatingWave length of the antenna, while the total height of the antenna may beof the order of one-sixteenth of its operating wave length. Further inaccordance with the present invention the antenna is provided with anadjustable impedance matching network, such as a reactive network, forcoupling the transmission line to the two antenna elements.

For a better understanding of the invention, to? gether with other andfurther objects thereof, reference is made to the following description,taken in connection with the accompanying drawing, and its scope will bepointed out in the appended claims.

In the accompanying drawing:

Fig. 1 is a cross sectional View of an antenna embodying the presentinvention;

Fig. 2 is a top plan view of the antenna of Fig. 1; and

Fig. 3 is a schematic electric diagram of the antenna of the inventionillustrating the adjustable impedance matching network for couplingV atransmission line to the antenna. y

Referring now to the drawing and particularly to Figs. l and 2, there isillustrated a high frequency antenna including lower antenna element Iand upper antenna element 2. As shown in Fig. 1, the lower antennaelement l is of substanfI tially conical shape. The upper antennaelement 2 has also been illustrated as being of substantially conicalshape, the apices of the two cones formed by antenna elements i and 2are arranged adjacent to each other. However, the shape of the upperantenna element 2 is not critical, and accordingly, antenna element 2may, for example, consist of `a flat disc instead of a cone..

Lower antenna element I is provided with horizontal supporting plate 3to which it may be secured by suitable rivets or screws indicated at 4.Thus, lower antenna element I and supporting plate 3 form an enclosedspace. Upper antenna element 2 is provided at its outer rim withreinforcing ring-shaped tube 5 to provide mechanical is connectedthrough coaxial transmission line S When high `frequency'translatorconsists of a transmitter, the antenna will radiate averticall-ypolarized wave having a substantially circular radiation pattern in ahorizontal plane. It

. will be noted. that antenna elements IV and 2 as illustrated consistof A'two very `flat cones while cone-2 vmay also consist of adiscso thatthe overall vertical height of the antenna may be rnade extremely small.Thus, the total height of the antenna need be no more than one-sixteenthof the operating wave length. The diameter of lower antenna element lshould preferably be slightly less than one-half the operating ywavelength, lwhile the diameter of upper antenna element 2 should beapproximately Vone-.half that of the lower antenna element I.

rEhe antenna as described has a low impedance whichis reactive.Accordingly, the radiation impedance of the antenna depends upon thefrequency of the wave intercepted or radiated. VIt is desirable,therefore, to provide a reactive impedancematching network betweencoaxial Ytransmission lline E andantennae I and 2 which should 'beadjustable .for dierent frequencies.

Y "The impedance matching network is 4preferably disposed in lowerantenna element i and further comprises means .for supporting the up-`per antenna element 42 from the 'lower element i This .constructionmakes it vpossible to support 'the two antenna elements in such a mannerthat a radiated wave does not forman interferenceV pattern which wouldbe the case if any .obstructions .such as insulating Vsupports werearranged in the space between elements Yl and .2.

For the `purpose oi electrically connecting outer conductor ID oftransmission line 6 to the lower Y .antennaelement l, there is providedcup-shaped Inner conductor 8 of L tion of outer sleeve "I5 is secured toupper antenna j element 2 through bushing plate YII'T- which con- Ajustthe capacitance Vof condenser ,30.

sists of a conducting material. Upper antenna element 2 is accordinglysupported by housing l2, cover I4 and outer sleeve I5.

Inner conductor 8 of transmission line 6 is connected to lower condenserplate `2 I'I as illustrated. Condenser plate is secured to housing I2through collar 2l which consists of an insulating material. .Condenserplate ..20 is thusfiixed to housing I2. Upper condenser plate 22 issecured to inner sleeve 23 which is slidably arranged in outer sleeve I5and consists of insulating material.

Upper Vcondenser plate 22 is electrically coupled to upper antennaelement 2 through inductance element 24 provided in the space formed byinner sleeve 23. Stud 25 which consists of a conducting material iselectrically connected to inductance element. "24 and may be screwedinto inner sleeve 23 or otherwise secured thereto. Stud 25 extendsthrough outer sleeve I5 and is screwed into bushing 265v which in turnis threaded into bushing yplate I1. Set screw '.21 is provided inbushing -26 for vlocking Vstud 25. Another set screwv 28 `is provided`in housing vI2 for `locking it to cover I4.

An impedance matching network is vthus provided for coupling coaxialtransmission line -6 to antennae I and 2. As show-n schematicallylinFig. 3, inner conductor 8 is'connected to ladjustable condenser 30 whichis represented in 1 by condenser plates 20 and v 22. Condenser -30 `-iscoupled to Vupper antenna element 2 through in"- ductance element 24yand stud 25 as shown rin Fig. 3. Another condenser 31 is providedbetween the junction point of condenser 3B and inductance element 24 on'the one hand `and housingY l2 on the other hand as `shownschema-tically in Fig. 3. Thus, condenser 3l -is effectively connectedbetween inner conductor 8 and lower antenna element I. Condenser 3|represents-the capacitance between condenser plate 22 and coverv I4.

It will now be seen that vcondensers L30 and 35| are individuallyadjustable. Thus, Yin order to adjust the capacitance of condenser 3|,set screw 2 is loosened and stud y25 is rotated thereby to move innersleeve 23 with respect 'to outer sleeve l5. Consequently rthe distancebetween condenser plate 22 and cover I4 maybe varied 'or adjusted. Itwill be obvious that by this adjustment the distance between condenserAplates '20 and 22 will be varied simultaneously. However, lthe distancebetween condenser plates 20 and 22 may be adjusted .by-loosening setscrew 23 and Y rotating upper antenna elementl which ,may tbe effected,'for example. by .gripping tube 5- .Accordnsly, upperantenna element.2.,outers1eeve l5, cover I4, inner `sleeve 123 and upper condenserplate y22 will rotate in .unison with respect to housing I2 thereby toadjust the Ydistance vbetween condenser plates .20 and .22. 'This inturn will .ad-

It will accordingly be evident that the .im-

pedance matching network including .condensers 3U, V3l and inductanceelement '24 .may beV adjusted while the antenna ,is in operation andtween condenser plates'20 and 22 onthe-one hand' `and between condenserplate 22 and cover I4-on, the other handiifthe antenna is Vintendedtooperate at va `predetermined frequency. In that case the adjustmentmay be effected while 'fthe 'antenna is assembled.

` Experiments carried out :with "the antennalof the invention haverevealed that the antenna has an eiciency comparable to that of aconventional vertical half-wave dipole antenna. Since the overall heightof the antenna may be as low as one-sixteenth of the operating wavelength, the antenna is particularly suitable for use on moving vehicleswhere a low clearance antenna is required. In spite of its small heightthe efliciency of the antenna is not impaired.

While there has been described what is at present considered thepreferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modications may be madetherein without departing from the invention, and it is, therefore,aimed in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

What is claimed is:

1. An antenna comprising a first antenna element, a second antennaelement, a line having first and second conductors, a housing forelectrically connecting said second conductor to said second element andfixed to said second element, a first condenser member connected to saidiirst conductor and xedly disposed in said housing, a second condensermember, means for electrically coupling said second condenser member tosaid first element, and movable means mounted on said housing andconnected to said second condenser member for adjusting the distance andthe capacitance between said condenser members, thereby to provide anadjustable impedance matching network between said antenna and saidline.

2. An antenna comprising a rst antenna element, a second antennaelement, a line having rst and second conductors, a housing forelectrically connecting said rst conductor to said second element andxed to said second element, a rst condenser plate connected to saidsecond conductor and iixedly disposed in said housing, a secondcondenser plate, a cover for said housing electrically connected to saidsecond element and insulated from said second condenser plate, saidsecond condenser plate being arranged to have its distance from saidcover adjusted to vary the capacitance between said second condensermember and said cover, said cover being arranged to be moved in unisonwith said second condenser plate for adjusting the distance andcapacitance between said condenser plates, thereby to provide anadjustable impedance matching network between said antenna and saidline.

3. An antenna comprising an upper antenna element, a lower antennaelement, a coaxial transmission line having an inner and an outerconductor, a housing for electrically connecting said outer conductor toone of said elements and disposed within said one element, a cover forsaid housing electrically connected thereto and mechanically connectedto and insulated from the other of said elements, a first condenserplate connected to said inner conductor and fixedly disposed in saidhousing, a second condenser plate electrically coupled to said firstcondenser plate and to said cover, means for electrically connectingsaid second condenser plate to said other element and for adjusting itsdistance from said cover to adjust the capacitance therebetween, andmeans for simultaneously moving said cover and said second condenserplate to adjust the distance and the capacitance between said condenserplates,

6 thereby to provide an adjustable impedance matching network betweensaid antenna and said transmission line.

4. An antenna comprising an upper antenna element, a lower antennaelement of substantially conical shape, the diameter of said lowerelement being larger than that of said upper element, a coaxialtransmission line having an inner and an outer conductor, a cupshapedhousing for electrically connecting said outer conductor to said lowerelement and disposed within said lower element, a cover threaded intosaid housing and electrically connected thereto, an insulating supportiiXed to said cover and to said upper element, a first condenser plateconnected to said inner conductor and iiXedly disposed in said housing,a second condenser plate electrically coupled to said rst condenserplate and to said cover, means for electrically connecting said secondcondenser plate to said upper element and for adjusting its distancefrom said cover to adjust the capacitance therebetween, thereby toadjust the distance and the capacitance between said condenser plates byrotating said upper element and said cover simultaneously with saidsecond condenser plate.

5. An antenna comprising an upper antennal element, a lower antennaelement of substantially conical shape, the diameter of said lowerelement being larger than that of said upper element, a coaxialtransmission line having an inner and an outer conductor, a cup-shapedhousing for electrically connecting said outer conductor to said lowerelement and disposed within said lower element, a cover threaded intosaid housing and electrically connected thereto. an insulating supportfixed to said cover and to said upper element, a first condenser plateconnected to said inner conductor and xed'ly disposed in said housing, asecond condenser plate electrically coupled to said first condenserplate and to said cover, a stud threaded into said upper element, aninductance element for electrically connecting said second condenserplate to said stud and to said upper element, an insulating member forsecuring said stud to said second condenser plate, thereby to adjust thedistance and the capacitance between said cover and said secondcondenser plate by rotating said stud and to adjust the distance and thecapacitance between said condenser plates by rotating said upper elementto rotate said cover simultaneously with said second condenser plate.

PAUL J. SELGIN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,031,103 George s- Feb. 1i8,1936 2,175,252 Carter Oct. 10, 1939 2,272,608 Hoiman Feb. 10, 19422,344,171 Rote Mar. 14, 1944 2,368,663 Kandoian Feb. 6, 1945 2,401,601Atwood June 4, 1946 FOREIGN PATENTS Number Country Date 114,368Australia Dec. 24, 1941 OTHER REFERENCES Three New Antenna Types andTheir Applications, by A. G. Kandoian, Proceedings of the IRE, February1946, pp. 70W-75W.

